Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item INTERACTIONS BETWEEN NITROGEN AND TEMPERATURE ON THE METABOLISM OF THE RED-TIDE MIXOTROPHIC DINOFLAGELLATE KARENIA SPP. IN SUPPORT OF PREDICTIVE MODELS: IMPLICATIONS FOR BLOOM DYNAMICS ON THE WEST FLORIDA SHELF(2023) Ahn, So Hyun; Glibert, Patricia; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The toxic mixotrophic dinoflagellate Karenia spp. forms blooms almost annually in the Gulf of Mexico, especially on the West Florida Shelf (WFS). Blooms typically initiate in early fall but can persist from months to years. Daily, Karenia vertically migrates to the surface water during the day, possibly experiencing changes in temperature, light, nitrogen (N), and prey type and availability. Therefore, this dissertation aimed to examine the interplay between Karenia’s photo-autotrophic and phago-mixotrophic metabolism and the short-term fluctuations in environmental conditions to understand how these factors may relate to the conditions under which Karenia spp. are found in the WFS.Title of Dissertation: INTERACTIONS BETWEEN NITROGEN AND TEMPERATURE ON THE METABOLISM OF THE RED-TIDE MIXOTROPHIC DINOFLAGELLATE KARENIA SPP. IN SUPPORT OF PREDICTIVE MODELS: IMPLICATIONS FOR BLOOM DYNAMICS ON THE WEST FLORIDA SHELF So Hyun (Sophia) Ahn, Doctor of Philosophy, 2023 Dissertation directed by: Professor Patricia M. Glibert, Marine Estuarine Environment Sciences A culture of K. mikimotoi balanced photon flux pressure (light availability) with consumption in overall metabolism when pulsed with 15N-NO3-, 15N-NH4+, or 15N-urea over the range of 15-25°C as shown by photosynthetic fluorescence. However, when shifted to 30°C, cells were significantly stressed, but urea-enriched cells showed a smaller decline in fluorescence, implying that urea might induce a photoprotective mechanism by increasing metabolic “pull.” Studies conducted with natural K. brevis winter and summer populations during 2021 showed that thermal history played a critical role. Unusually, summer blooms had higher biomass but were stressed photosynthetically and nutritionally. However, 15N-urea enriched summer cells had higher uptake rates as well as carbon (C) and N cell-1, especially in warmer waters, showing differential thermal responses based on N forms. Mixotrophy grazing measurements showed that K. brevis grazed both the picoplankter Synechococcus as well as the cryptophyte Rhodomonas. Grazing did not selectively target specific qualities of Synechococcus (based on differing N and P of the prey growth media), but ingestion rates were a function of prey-to-grazer ratios (R2=0.76) as well as prey amounts (R2=0.71). NanoSIMS confirmed 15N incorporation from Synechococcus in K. brevis. In natural communities of K. brevis, ingestion rates were also significantly related to prey-to-grazer ratios (p < 0.01) and by temperatures (p < 0.05) to a lesser degree (R2= 0.75) when incubated at ambient (24°C) and ambient temperature ± 5°C (19, 29°C). The grazer effects on the photosynthetic performance of grazer and prey were also examined. Grazing on Synechococcus indirectly reduce the photosynthetic performance of prey, especially at warmer temperatures but had little or no effect on the photosynthesis of K. brevis itself.Item Biomimetic Polymer Capsules: Novel Architecture and Properties(2021) Ahn, So Hyun; Raghavan, Srinivasa R.; Bentley, William E.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study focuses on polymer capsules made from biocompatible, water-soluble polymers. Typically, the capsule core is a hydrogel in which proteins, nanoparticles, or biological cells can be encapsulated, while the capsule shell is permeable to small, but not large molecules. We explore two new designs or architectures for such capsules. One is a multi-compartment capsule (MCC) where a capsule has several distinct compartments inside it. A second design is a multilayer capsule, where concentric layers of different chemistries surround a core. These new designs mimic structures commonly found in nature such as a eukaryotic cell or an onion. Our goal is to exploit these novel capsule architectures to achieve new or improved properties. In our first study, we introduce a new kind of multilayer capsule, wherein a protective shell of covalently crosslinked polymer (acrylate) surrounds a core formed by physical crosslinking (alginate). Alginate capsules are widely used for cell-encapsulation, but they are quite weak. We show that a covalent acrylate shell can be added to these capsules in a single step under mild conditions. The shell protects the core from degradation while allowing the encapsulated cells to remain viable and functional. A variation of the synthesis technique yields capsules with two concentric shells (alginate, then acrylate) surrounding a liquid core. Next, we create MCCs in which microbes from two different kingdoms, i.e., bacteria (Pseudomonas aeruginosa) and fungi (Candida albicans), are placed next to each other in distinct inner compartments. This MCC platform holds advantages over traditional co-culture as it eliminates physical contact between the two microbes and allows for real-time monitoring of cell growth in 3D. Using this platform, we study the effects of both physical variables (e.g., pH) as well as chemical additives (e.g., surfactants) on the growth of the two populations. We also detect crosstalk between the bacteria and fungi, i.e., as the bacteria grow, they inhibit the formation of hyphal filaments by the fungi, which make the fungi less invasive. Lastly, we create MCCs with ‘smart’ inner compartments, which are sensitive to various stimuli. An analogy is drawn to different organelles in a cell, which have different constituents and unique functions. We select the chemistry or architecture of each inner compartment of the MCC such that their responses are distinct and orthogonal. For example, one compartment alone breaks apart when the MCC is contacted with an enzyme, while another gets degraded by the introduction of hydrogen peroxide (H2O2), and a third is disrupted by ultraviolet (UV) light. Another concept is shown where the degradation of one compartment induces the degradation of another. We believe these new designs will make the MCC platform more attractive for various biological applications.